I know I shouldn't feel like this

I was talking to my sister, and her daughter is married to a type one diabetic.
Not for the first time the family went out together and suddenly everything had to be stopped, they needed to treat a hypo - but the diabetic had nothing with him, so everyone was looking for sweets or a shop, they had to abandon everything planned for the day, get back to the car, drive to where they could buy a drink - and then a meal and then wait to see what the BG level was and then there was an injection of insulin required so back to the car and wait and check - and by then it was too late to do anything and so they went home.

I know very well that a hypo can happen at any time, but as my sister said it is every time they want to go out, they have to wait until half the day has passed before leaving, and then there is some emergency - this time it was no hypo treatment, last time it was no insulin so they could not stay out and have a meal but had to drive home.

It seems that he gets into a panic when things go wrong, yet makes no plans for his own safety - he also gets really angry when questioned - my sister asked if he had his insulin with him this time, as he did not have it with him on their last trip out, and said that he was very short with her. He really is not a man at peace with himself.

My sister is now thinking that the only reason he married my niece was to have someone with him all the time - she can't get away from him to do anything at all. It is her birthday soon and her dad was asking her about what she wanted to do, and she was the centre of attention until her husband had 'a funny turn' - which seems to be when it happens most times. He was given sweets because this is not unexpected - but he needed to be got to somewhere he could have more to eat - so much more that he was injecting insulin.
My sister isn't stupid, but even my brother in law, who is a big gentle fellow and not exactly quick on the uptake has been remarking on the way these things happen.

This might sound particularly negative, but I've seen exactly this sort of thing in a friend of mine and her ex-husband.

My friend has a son with ASD, which her ex-husband doesn't understand (and refuses to try). Though they haven't been married for a decade, Ex-Hubby is constantly around, constantly complaining that my friend is "consumed" by their son's needs, and doesn't see that he's "that bad" (actually, their son is profoundly disabled, just not in a physical way).

Ex-Hubby has had a diagnosis of T1D for 30+ years. Around the time of their divorce, it suddenly got markedly worse - so much so that he had to give up work. As their son has grown older, Ex-Hubby has claimed that his T1D has "gotten worse", too. In fact, only his active participation in its control and management has gotten worse.

He says he's "too tired" to count carbs before he takes his insulin. He'll leave the house without either meds, recovery kit, phone or watch and then my friend will get a call at work from some cafe or restaurant asking her to come and get him because he's fainted and asked them to call her.

The morning of big appointments, he'll suddenly feel shaky. During family holidays (where my friend has to work intensely hard to manage their son), and which Ex-Hubby also goes in the name of Helping Out, he'll say he needs to go home because he's feeling unwell.

It's my view that Ex Hubby is not only very good at managing his T1D, but that he's managed to control it so precisely as to be able to steer himself into never-quite-actually-dangerous hypo situations when he's feeling a little neglected. To put this in context, in the 7 years I've known them, he's NEVER had a hypo when all has been going smoothly with their son - no matter what other stresses and strains were going on in his life.

I fell quite strongly that he's managed to position his T1D like a Sword of Damocles over my friend's head and uses it to coerce her into giving him attention. I find it incredibly immature and yet oddly admirable at the same time. I once talked to my friend about it, and shared my very stark view on how Ex Hubby is managing his T1D and she replied, "Of course he is. But what choice do I have but to answer every single call? I don't know how far he'll go, and if I let him go to hospital, [their son] would never get over it."

Using a life-threatening condition to control someone is a form of abuse. I'm certain on that. But whether that's what's happening to your niece... I don't know enough to say.